The invention relates to a method and a system for cleaning semiconductor elements, such as wafers or the like according to the preamble of the method claim or the device claim.
It is known that semiconductor slices are treated with liquid chemicals, in particular also ozonised, deionised (called DI in the following) water. The most varied systems for this purpose are known which comprise recirculation systems and so-called xe2x80x9csingle-passxe2x80x9d (one-way) systems. All of the systems have a container in which the semiconductor slices are received and through which the cleaning liquid flows, which comprises ozonised DI water and possibly other chemicals. The container can thereby be configured as an overflow tank, as a through-flow tank, as a rotary tank or the like and the supply of liquid can also be effected in the most varied manner, for example by being sprayed into the container via nozzles or being introduced via pipelines as a stream of liquid. In the case of recirculation methods, at least a part of the spent cleaning liquid is returned to circulation via filter and cleaning units, i.e. mixed with fresh ozonised DI water. The container is connected to a device for generating ozonised DI water via pipelines, in which device ozone, which is fed from an ozone generator, is dissolved in highly pure DI water.
In the case of such systems according to the state of the art, the ozone concentration in the ozonised DI water fluctuated and the inventors have set themselves the object of producing a method and a system for cleaning semiconductor elements by means of which a constantly high ozone concentration is achieved for the ozonised, deionised water which is used for cleaning.
This object is achieved according to the invention by the characterising features of the main claim and of the independent claim.
It is typical for the DI water provision in the semiconductor industry (also referred to as UPW=ultrapure water) to have extremely low conducting capacity (18 Mohm/cm), a low metal ion content ( less than 1 pp+/metal) and a small proportion of organic material (TOC (total organic carbon): less than 1 ppb), the DI water being neutral, i.e. the pH value is normally around 7.
It has been shown that the desired high ozone concentrations were not able to be produced for all of the highly pure waters used, for example, ozone concentrations of just 20 ppm were achieved, on the one hand, whereas 50 to 120 ppm were achieved on the other hand. In the case of low ozone concentrations it has been established furthermore that they depend only very little on the liquid flow volume while normally the ozone concentration increases when the flow volume becomes smaller. For example, in the case of flow volumes of 2 l/min, an ozone concentration of up to 150 ppm was achieved, with a flow volume of 10 l/min up to 70 ppm and with 20 l/min up to 40 ppm, while, in the other case with the same volumes, ozone concentrations respectively of 15 ppm, 10.5 ppm and below 20 ppm (not illustrated) were achieved.
Such a phenomenon is illustrated in FIG. 1, the characteristic lines show the ozone concentration relative to the through-flow of the DI water, xe2x80x9crow 1xe2x80x9d showing measured values for the expected ozone concentration and xe2x80x9crow 2xe2x80x9d showing measured values for an unexpected low ozone concentration.
It was hence shown that a significant ozone decomposition occurred although, because of using DI water of high purity, metal ions or metal oxides which catalyse the ozone decomposition, were not expected. The TOC value, which can be used as a measure for those substances which can reduce or consume the ozone by reaction with ozone, is small so that an appreciable loss was not expected.
The invention is therefore based on the surprising knowledge that, although it is not to be expected on the basis of expert knowledge, the decomposition rate of the ozone is increased in various DI waters. In FIG. 2, characteristic lines are indicated for the half-lives of ozonised liquids from the literature relative to the pH value. According to these data from the literature, the calculated half-life of the ozone decomposition is of the magnitude of approximately 1000 seconds, at a water temperature of 200xc2x0 C. and a pH value of 7.
In the case of another DI water, which fulfilled the same criteria as the DI water corresponding to the literature data, namely it contained few metal ions and low pressure and also a pH value between 6.8 and 5, the decomposition rate was high, the half-life was determined to be approximately 150 seconds, as is illustrated in FIG. 2 by the measured value xe2x80x9cCondition 1xe2x80x9d.
According to the invention, CO2 was added to the ozone/oxygen mixture generated by the ozone generator. By adding CO2 to the DI water, the decomposition rate was able to be reduced without substantially affecting the pH value and in fact there were achieved a half-life of approximately 750 seconds and an ozone concentration as is otherwise normal. This is shown in FIG. 2 by the measured value xe2x80x9cCondition 2xe2x80x9d, DI water with CO2 supplement, it being able to be detected that the half-life was able to be more than tripled relative to the measured value xe2x80x9cCondition 1xe2x80x9d.
FIG. 3 shows the ozone concentration at the outlet of the system for a through-flow of 10.75 l/min relative to the dosage of the DI water with CO2, the DI water without dosage showing the decomposition rate corresponding to xe2x80x9cCondition 2xe2x80x9d according to FIG. 2.
It can be detected from FIG. 3 that, with a dosage of less than 1% CO2, the system according to the invention could already deliver a threefold ozone concentration in comparison with the DI water which has no CO2 added. It is supposed that this behaviour can possibly be explained by the suppression of the radical decomposition chain of ozone, CO2 as xe2x80x9cScavengerxe2x80x9d slowing down the radical decomposition of ozone. It is supposed that traces of peroxides are present in the DI water which can possibly occur during UV treatment of DI water for the purpose of disinfection, when oxygen has not been completely removed during processing of the DI water. At the same time, a lowering of the pH value can be expected.
In total, the cleaning effect of semiconductor elements by means of ozonised, deionised ultrapure water can be stabilised by the method according to the invention and the system according to the invention since, because of the supply of CO2, the ozone concentration of the system according to the invention can be kept uniformly high even when using different DI waters.